System for tracking mill stand motor currents for optimizing the duty cycle

ABSTRACT

This disclosure relates to a system for automatically monitoring mill drive motor accelerating and load currents in the full field and weakened field range, for speed regulated drives with automatic cutoff of acceleration on reaching operator preset load conditions. The system enables obtaining maximum acceleration rates and maximum speeds compatible with motor overload ratings, so as to optimize the known duty cycle to make full utilization of the mill stand motor consistent with design specifications.

United States Patent Fludzinski [54] SYSTEM FOR TRACKING MILL STAND MOTOR CURRENTS r011 OPIIMIZING THE DUTY cvcu:

[ 51 Apr. 18 1972 Primary Examiner-Bemard A. Gilheany Posmorv FEEDBACK [72] Inventor: Jan M. Fludzinski, Williamsville,N.Y.

1 Assistant Examiner-W. E. Duncanson, Jr, [73] Assignee: Westinghouse Electric Corporation, Pitt- Attorney-EH. Henson, R. G. Brodahl and .l. J. Wood sburgh' 7 I 57 ABSTRACT [22] Filed: Aug. 17, 1970 l 1 I This disclosure relates to a system for automatically monitor- PP NW ,450 ing mill drive motor accelerating and-load currents in the full,

. field and weakened field range, for speed regulated drives with automatic cutoff of acceleration on reaching operator preset [52 I U. S. Cl. ..3l8/326,3l8/332 load conditions The system enables obtaining maximum [51] Ilii. Cl. ..H02p 5/00 celeration rates and maximum Speeds compatible with motor Field sfalcll 3/ overload ratings, so as to optimize the known duty cycle to 3 /26 make full utilization of the mill stand motor consistent with design specifications. [56] References Cited UNITED STATES PATENTS 5 Claims, 3 Drawing Figures 3,416,058 12/1968 Hill et al. ..3l8/326 I COMPUTER I INPUT v 13%? I2 a I r a E M c i iiir 9 I N g gi i {26 28 l 4 2o EED 'g''g Z 4 REGULATOR 58%{5 COMMAND I 3 i DIFFERENTIATION- l MULTIPLIER CIRCUIT DRIVER v w PATENTEUAPR 18 1912 SHEET 2 UF 2 CURRENT LIMITING STAND LOAD RATE l llO lOO-- RP M.

FIG. 3

SYSTEM FOR TRACKING MILL STAND MOTOR CURRENTS FOR OPTIMIZING THE DUTY CYCLE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a system for tracking mill stand motor currents for the purpose of optimizing loadings on a mill stand motor, consistent with the known duty cycle prescribed by the manufacturer.

2. Description of the Prior Art In rolling mills, the operator frequently does not make maximum utilization of the mill stand motor. This may be accounted for in large measure by the fact that the rolling mill schedule frequently does not require 100 percent loading for large portions of the working day. This may arise from a variety of conditions which need not concern us at this time. Mill stand motors are rated for overload conditions by the manufacturer. For example, a motor may be rated at 125 percent overload for 2 hours, and 175 percent maximum overload for frequently repeated momentary loads. Where the mill has not been used to 100 percent capacity, the result is that the operator may without any danger of overheating, overload the mill stand motor to increase work output.

Less than full capacity conditions perennially arise in rolling operations, and the mill operators response therefore is to speed up and temporarily overload the motor in order to augment production efficiency. However, in the prior art, overloading has been a more or less cut-and-dry arrangement, because the operator had no way of monitoring the motor load current since his instrumentation displays only total current, with the result that he may stop accelerating too early resulting in a more or less fortuitous approach toward optimizing the duty cycle.

SUMMARY OF THE INVENTION In accordance with the principles of the invention there is provided a system for tracking currents in a mill motor operating in the weakened field range between base and maximum speeds, for the purpose of optimizing the duty cycle under constant inertial conditions. Means coupled to the mill motor, provide a first signal which is a function of the preselected acceleration ramp current for the mill motor. Means also provide a plurality of signals which are respectively, a function of the instantaneous magnitude of the total mill motor current. Algebraic summation means, operatively connected to the first signal means and the plurality signal means, provide a succession of summed output signals, each discrete summed output signal being a function of the'instantaneous motor load current. Additionally, means provide a'selectable predetermined percentage of the instantaneous motor load current. Further, means are adapted to receive the discrete summed output signal and the predetermined percentage of instantaneous motor load current for the purpose of comparison and for delivering an inhibit signal when the compared magnitudes are equal. Finally, means receive the inhibit signal and terminate the preselected acceleration ramp current at a steady state magnitude for application as a speed reference to the mill motor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing the mill stand motor current tracking system in accordance with the invention;

FIG. 2 is a diagram depicting the relationship between flux and speed in the region between base speed and maximum speed; and

FIG. 3 is a diagram of the percentage load torque versus rpm showing various acceleration rates in relation to stand load current for a given load.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT Referring now to FIG. 1, the instant invention is intended to be practiced in the environment of a cold rolling mill indicated generally at 10, comprises an upper roller member 12, and a lower roller member 14 operative to longitudinally displace a strip of material 16. The material 16 is to be reduced in the rolling mill, the thickness between the rollers 12 and 14 being regulated by means of a screwdown motor 18. Since the actuation of the screwdown motor 18 fonns no part of this invention, the remaining details concerning its actuation are omitted.

A mill motor 20 is mechanically coupled to the roller members 12 and 14 or to 12A and 14A to control displacement of the strip of material 16 through the mill. The mill motor 20 is physically large than the screwdown motor 18, and to it falls the burden of reducing the material. The speed of the mill motor 20 is selected in logic circuitry 22 which sends a mill speed command to a ramp function generator 24 which generates a speed reference to for application to a speed regulator indicated generally at 26. The output signal of the speed regulator 26 is applied to a power supply for the motor indicated generally at 28. The power supply 28 may be a thyristor power supply or a generator power supply. A tachometer generator 30 is coupled to the shaft of the mill motor 20 to generate a feedback signal or which is fed back to the speed regulator 26.

The circuitry for deriving the accelerating current signal is indicated generally at 32. The circuitry may utilize the motor operated potentiometer to be described or it may be a static multiplier/divider.

The speed reference signal on is fed to a driver amplifier 34 which is connected to motor operated potentiometer having a motor identified at 36 and a number of ganged parallel plate potentiometers two of which are identified at 38 and 40 respectively. The wiper 42 of the potentiometer 38 is connected to provide a position feedback signal to the drive amplifier 34.

The signal 0) is also applied to a difierentiation-multiplier circuit 44, the output of which provides a signal K0) which is connected across the potentiometer 40. The output of the potentiometer 40 is connected to a summation amplifier indicated generally at 46. Another input to the summation amplifier 46, is the total current I, which is derived from the shunt resistor 48 in the motor armature circuit of the mill moor 20. The output of the summation amplifier I, is connected to a load percentage selection network indicated generally at 48; this circuitry comprises an amplifier proper at 50, with a feedback loop having a plurality of resistors 52, 54, 56 and so forth, serially connected with open contacts identified at 58, 60, 62 respectively. As indicated by the dotted lines the selective closure of the contacts 58, 60, 62 is made through the logic circuitry 22. A contact relay 64 is connected in the output of the amplifier 50, with the cooperating contacts being within the logic circuitry 22.

OPERATION OF THE CIRCUITRY Cold rolling mills are seldom operating so that the mill speed motor is operating up to percent load. Usually scheduling delays and other considerations prevent 100 percent utilization of the motor at all times. The operator may therefore overload the motor at other times (within its rated capacity) so as to provide more efficient utilization. This may advantageously be done for example, in order to reduce the number of passes of the material 16 through the rollers l2, 14. Mill speed motors are frequently rated by the manufacturer at percent overload for 2 hours and percent overload for maximum frequently repeated momentary loads. If during rolling the load were consistently 100 percent, this would represent 100 percent efficiency. Since the mill stand motors are not run at 100 percent load, it is therefore possible to overload them within the rating capabilities set forth by the manufacture, and therefore achieve reductions in a faster period of time. The mill operator can therefore accelerate faster and accomplish the job at a faster rate.

In operation, the DC motor 20 is runup to base speed w As indicated-in FIG. 2 the counter EMF (CEMF) is equal to Kdxo. During the time that the motor is run up to base speed counter EMF constantly increases and the flux remains constant. As indicated in FIG. 3, I, that is the load current, is constant while the motor is running up to base speed w The monitoring system of FIG. 1 only comes into operation in the field weakening range from w to w, (the maximum speed). When base speed a), is reached the counter EMF is kept constant and the field is weakened in order to increase the speed (FIG. 2). More current is now needed to obtain the same torque. lnthe weakened field region, the load current 11 is a function only of the speed-it does not depend upon the acceleration rate. The higher the speed the more load current I i is required for the same load and for the same reduction. The important parameter then as far as the mill operator is concerned is the load current 1,; however, his instrumentation is such that only I the total current is displayed. Therefore, if he observes only the total current, he hasno way of knowing where to stop, and most probably he will stop short of the overload condition which he has selected. The important parameter then is 11 the load current.

The expression for motor load current may be derived as follows:

T,,,=T +T, 1 where T, motor torque T, load torque T acceleration torque (dw/dr) (2) where J= moment of inertia (motor load) w= rotational speed do) m Tll', dt Tm Kidlli. where K a constant (I) motor field flux -I, armature current a 1 a 5) where K a constant a acceleration current Solving Equation (5) for I a a/ 14) T; I, +1 7 where I, armature current I IV load current 1,, acceleration current Solving for 1' II t la Substituting Equation (6) for Equation (8) 1! I: 11 Km Substituting for T.. from equation (2) J dw I,I dt (10) Between base speed 0),, and maximum speed m the field flux is weakened and the motor counter EMF E is kept constant E e 1 1) where K, a constant Solving for (b in Equation l 1),

e l Substituting the relationship for 4) (Equation 12) in Equation l0) J da) The constants may be combined into a new constant K. Additionally let to dw/dt I1=I.K J% w 14 Referring now back to FIG. 1 the speed reference a) is fed to the driven amplifier 34, the output of which drives the motor 36. The voltage E connected to potentiometer 38, is proportional to the counter EMF which is constant within the weakened field of operation (0,, w As indicated on the drawing by the caption POSITION FEEDBACK, a signal of opposite polarity to the speed reference signal to is fed back to the input of the amplifier 34. When these two signals are equal, the input to the amplifier 34 is zero, and the motor 36 stops. Since the potentiometers 38 and 40 are ganged, each potentiometer has been displaced a distance a.

to (1E 1 5 a w/E 1 E/Kw the motor counter EMF equation where 4) flux E counter EMF w= speed in radians or rpm K a constant 1/= /E) (18) Comparing Equations (16) and l 8) The signal to is differentiated in circuit 44 and multiplied by a constant K. Therefore the wiper of potentiometer 40 is equivalent to a Kw/ 292 I u m X to (21) Since J/K is a constant K The amplifier 46 performs an algebraic summation, I, being derived from shunt 48. I

I! t a The system for tracking I and 1,, appears in FIG. 1.

A complete run down of the operation of the system will serve to provide an overall and comprehensive understanding of the system. The mill may have been running at perhaps percent load, and a job now comes through the mill, and the operator decides to run at perhaps say percent load. The logic circuitry in response to a manual or a computer command selects the mill speed, and a mill speed command signal is sent to the ramp function generator 24, which generates a ramp appropriate of slope which is fed to the speed regulator 26. The actual speed of the motor 20 is monitored by the tachometer 30 which feeds back a signal to the speed regulator 26. When the inputs to the speed regulator 26 are equal, the motor will then be running at the correct selected speed. If the operator decides that he now desires to run at 1 10 percent overload, (see FIG. 3) the logic circuitry then closes the appropriate contacts in the load selection network 48. Let us assume for example that this means that contacts 58 will be closed and resistor 52 is then in the feedback path. This in effect determines the amount of load current feedback for the overload selected. The accelerating current will be determined by the circuitry 32 which supplies an accelerating current 1,, to the summation amplifier 46. The total motor current I is derived from the shunt sensor 48 which is applied as I, to the have a choice of a number of acceleration rates as indicated by the captions Rate 1, Rate 2, and Rate .3..Suppose for example, that Rate 1 had been selected. The load percentage selection network 48 would deenergize the contact relay 64 when I had reached point b; I, would be at point a. If Rate 2 or Rate 3 had been selected, I, would reach points 0 or d respectively,

but I, would still be held at point b the correct value for 110 percent load.

it will therefore be apparent that there has been disclosed a system for monitoring mill drive motor currents which has wide application for industrial use.

I claim as my invention:

1. A system for tracking currents in a mill motor operating in the weakened field range between base and maximum speeds, for. optimizing the duty cycle under constant inertial conditions, comprising:

a. means coupled to said mill motor for providing a first signal which is a function of the preselected acceleration ramp current for said mill motor;

b. means for providing a plurality of signals which are respectively, a function of the instantaneous magnitude of the total mill motor current;

. means for algebraic summation, operatively connected to said first signal means and said plurality signals means, to provide a succession of summed output signals, each discrete summed output signal being a function of the instantaneous motor load current;

. means for providing a selectable predetermined percentage of said instantaneous motor load current;

signal and said predetermined percentage of instantaneous motor load current for comparison, for delivering an inhibit signal when the respective magnitudes are equal, and

f. means for receiving said inhibit signal for terminating said preselected acceleration ramp current at a steady-state magnitude for application as a speed reference to said mill motor.

2. A system according to claim 1 where, in said means coupled to said mill motor, said first signal is equal to:

K a constant of proportionality o) the acceleration of said mill motor 45 the instantaneous motor flux both in units compatible with the standard of measurement selected.

3. A system according to claim 1 wherein said means coupled to said mill motor comprises logic circuit means and ramp function generator means, said logic circuit means being coupled to said ramp function generator means for providing a selection command for said preselected acceleration ramp current, and adapted to receive said inhibit signal to further command said ramp function generator means to deliver said means adapted to receive said discrete summed output steady-state magnitude speed reference for said mill motor. v

4. A system according to claim 1 wherein said means adapted to receive said discrete summed output sig'nalsand said predetermined percentage of instantaneous motor load current comprises feedback amplifier means having an input to receive said discrete summed output and a feedback path which feeds back said predetermined percentage in opposite polarity for algebraic addition with said discrete summed output signals.

5. A system for tracking currents in a mill motor operating in the weakened field range between base and maximum speeds, for optimizing the duty cycle under constant inertial conditions, comprising:

a. means coupled to said mill motor for providing a first signal which is a function of the preselected acceleration ramp current for said mill motor, comprising driver amplifier means, a second motor, and dual po entiometers mechanically ganged to said second motor so that their respective'wipers are displaced in unison, the first of said potentiometers being connected across a source of constant potential which is proportional to the mill motor counter EMF in said weakened field, the position of the wiper of the first potentiometer being a function of 11, said latter wiper being connected as one input to said driver amplifier-means, a second input to said driver amplifier means being said preselected ramp speed electrical signal, the output of the driver amplifier means, energizing said second motor, the second of said potentiometers being connected .across an additional potential source which is proportional to K6), the rotary displacement of said second motor being arrested when the inputs to the driver amplifier means are equal, the wiper of said second potentiometer being electrically proportional to Kai/(b where K a constant of proportionality a: the acceleration of the mill motor the instantaneous motor flux both in units compatible with the standard of measurement selected;

b. means for providing a plurality of signals which are respectively, a function of the instantaneous magnitude of the total mill motor current;

. means for algebraic summation, operatively connected to said first signal means and said plurality signal means, to provide a succession of summed output signals, each discrete summed output signal being a function of the instantaneous motor load current;

. means for providing a selectable predetermined percentage of said instantaneous motor load current;

. means adapted to receive said discrete summed output signal and said predetermined percentage of instantaneous motor load current for comparison, for delivering an inhibit signal when the respective magnitudes are equal, and

f. means for receiving said inhibit signal for terminating said preselected acceleration ramp current at a steady-state magnitude for application as a speed reference to said mill motor. 

1. A system for tracking currents in a mill motor operating in the weakened field range between base and maximum speeds, for optimizing the duty cycle under constant inertial conditions, comprising: a. means coupled to said mill motor for providing a first signal which is a function of the preselected acceleration ramp current for said mill motor; b. means for providing a plurality of signals which are respectively, a function of the instantaneous magnitude of the total mill motor current; c. means for algebraic summation, operatively connected to said first signal means and said plurality signals means, to provide a succession of summed output signals, each discrete summed output signal being a function of the instantaneous motor load current; d. means for providing a selectable predetermined percentage of said instantaneous motor load current; e. means adapted to receive said discrete summed output signal and said predetermined percentage of instantaneous motor load current for comparison, for delivering an inhibit signal when the respective magnitudes are equal, and f. means for receiving said inhibit signal for terminating said preselected acceleration ramp current at a steady-state magnitude for application as a speed reference to said mill motor.
 2. A system according to claim 1 where, in said means coupled to said mill motor, said first signal is equal to: K omega / phi where K a constant of proportionality omega the acceleration of said mill motor phi the instantaneous motor flux both in units compatible with the standard of measurement selected.
 3. A system according to claim 1 wherein said means coupled to said mill motor comprises logic circuit means and ramp function generator means, said logic circuit means being coupled to said ramp function generator means for providing a selection command for said preselected acceleration ramp current, and adapted to receive said inhibit signal to further command said ramp function generator means to deliver said steady-state magnitude speed reference for said mill motor.
 4. A system according to claim 1 wherein said means adapted to receive said discrete summed output signals and said predetermined percentage of instantaneous motor load current comprises feedback amplifier means having an input to receive said discrete summed output and a feedback path which feeds back said predetermined percentage in opposite polarity for algebraic addition with said discrete summed output signals.
 5. A system for tracking currents in a mill motor operating in the weakened field range between base and maximum speeds, for optimizing the duty cycle under constant inertial conditions, comprising: a. means coupled to said mill motor for providing a first signal which is a function of the preselected acceleration ramp current for said mill motor, comprising driver amplifier means, a second motor, and dual potentiometers mechanically ganged to said second motor so thaT their respective wipers are displaced in unison, the first of said potentiometers being connected across a source of constant potential which is proportional to the mill motor counter EMF in said weakened field, the position of the wiper of the first potentiometer being a function of 1/ phi , said latter wiper being connected as one input to said driver amplifier means, a second input to said driver amplifier means being said preselected ramp speed electrical signal, the output of the driver amplifier means, energizing said second motor, the second of said potentiometers being connected across an additional potential source which is proportional to K omega , the rotary displacement of said second motor being arrested when the inputs to the driver amplifier means are equal, the wiper of said second potentiometer being electrically proportional to K omega / phi where K a constant of proportionality omega the acceleration of the mill motor phi the instantaneous motor flux both in units compatible with the standard of measurement selected; b. means for providing a plurality of signals which are respectively, a function of the instantaneous magnitude of the total mill motor current; c. means for algebraic summation, operatively connected to said first signal means and said plurality signal means, to provide a succession of summed output signals, each discrete summed output signal being a function of the instantaneous motor load current; d. means for providing a selectable predetermined percentage of said instantaneous motor load current; e. means adapted to receive said discrete summed output signal and said predetermined percentage of instantaneous motor load current for comparison, for delivering an inhibit signal when the respective magnitudes are equal, and f. means for receiving said inhibit signal for terminating said preselected acceleration ramp current at a steady-state magnitude for application as a speed reference to said mill motor. 